Automatic platen gap adjusting apparatus

Abstract: An automatic platen gap adjusting apparatus is provided with a stepping motor (5) for moving a carriage (1) loaded with a recording head (2) in the vertical direction of a platen (4), a rotary encoder (12) for producing a plurality of pulse signals proportional to the rotational speed of the motor, the rotary encoder being equipped with a detection mark on its circumference, a time difference integrating device for obtaining the integrated value of the time difference between the pulse signal from the rotary encoder and the driving pulse of the stepping motor (5) by moving the carrier (1) from a reference position in the direction of the platen (4), and an abutment decision device for detecting whether the value thus obtained has reached a predetermined value. The thickness of the paper is calculated by a paper thickness calculation device according to the number of pulses produced up to a point of time the signal is produced from the abutment decision device. The apparatus of the present invention can automatically adjust the relative gap (G) between the platen (4) and the recording head (1) with precision while preventing press marks from appearing on the paper as much as possible because the paper is caused to abut against the carriage.

Description: The present invention relates to an apparatus for automatically adjusting the gap between a platen and a printing head in proportion to the corresponding thickness of recording paper. With recording heads, in order to effect high-speed printing, the distance between a platen and the printing head should be as small as possible. This applies particularly to wire dot recording heads for use in printing characters by striking wires against recording paper via ink ribbons, where the striking stroke of the wire have to be set as small as possible. On the other hand, it is desirable that many kinds of recording paper are printable, leading to the need that the distance between the recording head and recording paper tends to vary to a greater extent. This applies especially to mechanically strong recording heads of the wire dot type which render it possible to make copies with the aid of a copying material. For this reason, especially printers using a wire dot recording head have been provided with a mechanism of adjusting the relative gap between the platen and the recording head. However, the problem is that the work of selecting the optimum gap for specific recording paper requires a great deal of skill and is also troublesome. In order to solve the problem above, Examined Japanese Patent Publication No. Hei. 4-14634, for instance, discloses a printer providing an encoder for producing pulse signals corresponding to the movement of a carriage from the initial position and a control unit for processing feedback pulse signals from the encoder. In this case, when a recording head abuts against recording paper, the fact that a pulse motor for driving the carriage starts stepping out is detected from variations of the number of pulses from the encoder, whereby the thickness of the recording paper is obtained from the movement of the carriage up to the abutment point of time, so that the carriage position is made controllable according to the data thus obtained. Notwithstanding, there still arises a problem in that, since the recording head is forced to abut against the recording paper until the pulse motor starts to step out, recording paper such as copying paper which colors in response to external force may bear press marks. Prior art document PATENT ABSTRACT OF JAPAN JP-A-03 169 665 discloses a platen gap adjusting apparatus in which a printing head is driven under CPU control to contact a platen by operation of a stepping motor. When the printing head contacts the platen, the time interval of a rectangular wave changes and the CPU stops movement of the head. A resulting value is stored. Paper is then brought into position and the head is moved until it contacts the paper. A second value is thus produced. The paper thickness, which is the difference between the first stored value and the second produced value, is determined and used to set a platen gap. This approach is similar to that discussed, where the print head is brought into contact with the paper by a stepping motor, where a step pulse may be supplied to the stepping motor when the paper is less than a full step away from contacting the head. Thus, the head may be overdriven into the paper, thereby marring the surface of the paper. In view of the foregoing problems, an object of the present invention is to provide an automatic platen gap adjusting apparatus capable of accurately measuring the corresponding thickness of recording paper by only forcing a carriage to contact the recording paper with minimum required force. In order to solve the foregoing problems, an automatic platen gap adjusting apparatus according to independent claim 1 is provided. Further advantageous features, aspects and details of the invention are evident from the dependent claims, the description and the drawings. The automatic platen gap adjusting apparatus comprises especially a stepping motor for moving a carriage loaded with a recording head in the vertical direction of a platen shaft, a movement detection device for outputting pulse signals proportional in number to the movement of the carriage, an abutment decision device for integrating, by moving the carriage from a reference position in the direction of a platen, the time difference between each stepping-motor driving pulse and the pulse signal produced from the movement detection device due to driving by the driving pulse so that it is detected whether the integrated time difference value has reached a predetermined value, a paper thickness calculation device for calculating the corresponding thickness of recording paper on the basis of the movement of the carriage from the reference position, and a control device for adjusting the relative gap between the carriage and the platen by driving the stepping motor according to the data given by the paper thickness calculation device. With the automatic platen gap adjusting apparatus, while the movement of the carriage from the reference position in the direction of the platen is calculated, the time difference between the stepping-motor driving pulse and the pulse signal of the movement detection device is detected. When the integrated time difference value reaches the preset value, it is decided that the recording head has abutted against the recording paper before the stepping motor steps out. The corresponding thickness of the recording paper is calculated from the movement of the carriage until then. The relative gap between the platen and the carriage is thus set at the optimum value. In the accompanying drawings: Fig. 1 is a block diagram illustrating an embodiment of the present invention; Fig. 2 is a diagram illustrating the construction of the periphery of a gap adjusting mechanism according to the present invention applied to a serial printer; Fig. 3 is a flowchart showing the operation of a platen gap adjusting apparatus of the present invention; Fig. 4 is a waveform chart illustrating the operation of the apparatus of Fig. 3; Fig. 5 is a diagram illustrating the relation between kinds of paper and the number of driving pulses; Fig. 6 is a block diagram illustrating another embodiment of the present invention; and Fig. 7 is a block diagram illustrating still another embodiment of the present invention. Referring to the accompanying drawings, a detailed description will subsequently be given of an embodiment of the present invention. Fig. 2 illustrates a mechanism embodying the present invention for adjusting the relative gap between a platen and a recording head exemplified in a serial printer. In Fig. 2, numeral 1 denotes a carriage which is loaded with a recording head 2, provided on a guide shaft 3 rotatably fitted to the base in an eccentric condition and constructed so that it can adjust the relative gap G between the platen 4 and the recording head 2. Further, numeral 5 denotes a stepping motor which is driven in 2-2 phase excited state with a period of 3.5 milliseconds and coupled via a reduction gear 6 to a driven wheel 7 on the guide shaft 3. Moreover, a rotary encoder 12 is employed as a movement detection device. A code plate 10 of the rotary encoder 12 is fitted to a shaft 9 for the purpose of outputting pulse signals proportional in number to an angle of rotation. The code pattern of the code plate 10 has been selected so that one pulse width is output in synchronization with the single-phase driving of the stepping motor 5. Further, numeral 11 denotes a position detector for detecting a reference position, for example, a micro-switch, which is set where the carriage 1 is situated away at the farthest from the platen 4. Further, numeral 13 denotes a code detection device forming the rotary encoder. Numeral 15 denotes a guide member for supporting the other end of the carriage 1. Signals from these position detector 11 and rotary encoder 12 are applied to a controller 16 and used to obtain the optimum gap for the corresponding thickness of recording paper. The controller 16 drives the stepping motor 5 to move the carriage 1 in the direction of an arrow A in Fig. 2, thus adjusting the gap G between the recording head 2 and the platen 4. Fig. 1 illustrates an example of the controller 16. In Fig. 1, numeral 20 denotes a control device for controlling the rotation of the stepping motor. When a loading switch (not shown) is depressed, the control device 20 causes the carriage 1 to set at the reference position by moving away from the platen 4 until a signal is sent out of the position detector 11. Subsequently, the carriage 1 is moved in the direction of the platen so that the recording head 2 is caused to abut against the recording paper. After the thickness of the recording paper is detected, the stepping motor 5 is driven so that the optimum gap is obtained for the recording paper thus loaded. Further, numeral 21 denotes a time difference detection device for detecting whether a change is arisen in the time difference between the pulse signal period from the rotary encoder and the driving period of the stepping motor 5 so as to output ΔTn. Further, numeral 22 denotes a time difference integrating device for calculating an integrated value Σ(ΔTn) of the ΔTn and supplying the result to an abutment decision device 23 which will be described below. As noted above, numeral 23 denotes the abutment decision device which is adapted to output a signal when the integrated time difference value given by the time difference integrating device 22 coincides with a preset value which is, in this embodiment, not greater than 1/2 of the period of the stepping-motor driving pulse, for example, 1.5 milliseconds. In the case where the integrated time difference value is in the range of not greater than 1/2 of the stepping-motor driving period, the stepping motor 5 increases an output torque in accordance with an increase of the integrated time difference value. Therefore, the stepping motor 5 does not step out. Further, numeral 24 denotes a paper thickness calculation device for calculating the pulse signal from the rotary encoder 13 in accordance with the signal from the position detector 11 and for stopping the calculating operation in accordance with the signal from the abutment decision device 23 so as to calculate the thickness of the recording paper on the basis of the number of pulse signals. Referring to a flowchart of Fig. 3, the operation of the apparatus thus constructed will subsequently be described. When a loading switch (not shown) is depressed to load the recording paper (Step i), for the purpose of distributing an influence due to a parallel adjustment error between the platen 4 and the guide shaft 3 uniformly to both the left and right sides of the printing region, the control device 20 moves the carriage 1 to the substantially center of the printing region (Step ii). Then the stepping motor 5 is inversely driven to move the carrier 1 in the direction of the position detector 11 (Step iii). The stepping motor 5 is stopped when the signal is produced from the position detector 11, that is, the carriage 1 is set at the reference position (Step iv). Then the control device 20 positively drives the stepping motor 5 at a predetermined speed to move the carriage 1 in the direction of the platen 4. Simultaneously, the control device 20 counts the number of pulse signals from the rotary encoder. Incidentally, since the stepping motor 5 tends to operate irregularly when the carriage is initially moved, it is preferred to stop detecting the time difference between the pulse signal of the rotary encoder 12 and the driving pulse of the stepping motor 5 to the extent that the stepping motor 5 rotates by approximately 30-pulse times. After the carriage is thus moved by the predetermined number of pulses, the time difference detection device 21 detects an occurrence of the time difference between the driving pulse period of the stepping motor 5 and the pulse signal period of the rotary encoder 12. As shown in Fig. 4, since a load acting on the stepping motor 5 is constant until the recording head 2 abuts against the recording paper, the integrated value of the time difference between each pulse signal period of the rotary encoder 12 and the driving pulse period of the stepping motor 5 is zero during this time (Step v). When the recording head 2 abuts against the recording paper in this manner, the load is applied to the carriage 1, so that the speed of the rotary encoder 12 decreases, and the pulse signal period slows, whereby the time difference ΔTn is produced. The time difference integrating device 22 sequentially integrates the time differences ΔTn, and then supplies the results to the abutment decision device 23. The load thus increases as the carriage 1 moves closer to the platen, thus causing the integrated value Σ(ΔTn) of the time difference to sharply increase. When the integrated value Σ(ΔTn) reaches the preset value (Step vi), the abutment decision device 23 decides that the recording head 2 has abutted against the recording paper, and outputs a signal. On receiving the signal from the abutment decision device 23, the control device 20 stops the operation of the stepping motor 5, so that the carriage 1 is stopped to move. Further, the paper thickness calculation device 24 stops counting the number of pulses from the rotary encoder 12, and calculates the thickness of the paper (Step vii). The control device 20 drives the stepping motor 5 to make the carriage 1 move in the direction of the arrow A in Fig. 2 up to the optimum position, so that the gap is adjusted to an optimum value on the basis of the calculated thickness of the paper (Step viii). According to the embodiment of the present invention, although the carriage 1 has actually abutted against the recording paper, the abutment of the carriage 1 is decided before the stepping motor 5 steps out. Since any useless driving pulses are not supplied to the stepping motor 5, there is no fear of putting press marks on the recording paper. Fig. 5 shows the form of an increase in the integrated value Σ(ΔTn) of the time difference after the recording head 2 has started abutting against the recording paper. As shown in Fig. 5 (particularly, curve A), in the case of hard paper such as kraft paper, the value rapidly increases because the paper is substantially free from deformation even though it is pressed by the recording head 2. Further, as shown in Fig. 5 (particularly, curve B), in the case of a plurality of stacked sheets of copying paper, the value relatively increases gently because the copying paper is slightly deformed by which the recording head 2 presses. If that form of a change in the number of pulses is employed as decision-making information, it may be possible to set the gap by taking into consideration not only the thickness but also the quality of the recording paper, so that printing of higher quality becomes feasible. Although the reference position detection device 11 is used to determine the reference position in the aforesaid embodiment of the present invention, similar effect is made obviously achievable by applying the abutment decision method above to measuring the distance between any position of the carriage and the surface of the platen, then moving back the carriage up to the original position and obtaining the distance up to a position where the carriage abuts against the platen loaded with recording paper. More specifically, a reference position detecting device 25 is provided as shown in Fig. 6. The carriage is moved in the direction of the platen to detect the time a signal is produced from the abutment decision device 23. Then the carriage is moved back by a predetermined distance from the surface of the platen and the reference position detecting device 25 is caused to store the distance as the number of driving pulses. The recording paper is subsequently loaded. As in the case of the preceding embodiment, operational steps are followed on the basis of the data from the reference position detecting device 25, whereby the thickness of the recording paper can be detected. Further, similar effect is made obviously achievable according to Fig. 7 by providing devices for driving the carriage 1, for example, a bumper 7a for the driven wheel 7 and a bumper settling pin 30 on the static side as shown in Fig. 7, and making use of these devices for mechanically setting the reference position. Although the rotary encoder 12 fitted to the stepping motor 5 is used to detect the movement of the carriage 1 in the direction of the platen according to the present embodiment, similar effect is made obviously achievable by providing, between the carriage 1 and the base, a displacement measurement device such as a magnet scale for converting the linear distance into the number of pulses. Moreover, although it has been arranged that the driving signal of the stepping motor for driving the carriage is synchronized with the pulse signal of the rotary encoder according to the present invention, similar effect is made obviously achievable by using the number of driving pulses contained in the pulse signal period to decide the moving state of the carriage when the signal period is largely shorter than the period of the pulse signal of the rotary encoder 12. Still further, the special stepping motor is used to move the carriage in the vertical direction of the platen shaft according the embodiment above. However, such a special one can be dispensed with provided the carriage is interlocked via a transmission mechanism with a stepping motor for reciprocating the carriage during the printing operation. As set forth above, the automatic platen gap adjusting apparatus according to a preferred aspect of the present invention is provided with the stepping motor for moving the carriage loaded with the recording head in the vertical direction of the platen shaft, the movement detection device for outputting pulse signals proportional in number to the movement of the carriage, the abutment decision device for integrating the time difference between each stepping-motor driving pulse and the pulse signal originated from the driving operation based on the driving pulse and produced from the movement detection device by moving the carriage from the reference position in the direction of the platen so as to detect that the integrated time difference value has reached the predetermined value, the paper thickness calculation device for calculating the corresponding thickness of recording paper on the basis of the movement of the carriage from the reference position, and the control device for adjusting the relative gap between the carriage and the platen by driving the stepping motor according to the data given by the paper thickness calculation device. Consequently, the abutment point of time can be decided without pressing the recording paper against the platen with needlessly great force and press marks are also prevented from being left on the recording paper. Moreover, the abutment point of time can be decided precisely as unstable behavior at that time can be dealt with statistically. While not only the thickness but also quality of recording paper is taken into consideration, further, the relative gap between the platen and the recording head can be set by deciding an increasing rate of the integrated value of time so as to determined the corresponding kind of recording paper.

Claims: An automatic platen gap adjusting apparatus comprising : a stepping motor (5) for moving a carriage (1) loaded with a recording head (2) towards and/or from a platen (4); a movement detection means (12) for outputting pulse signals proportional in number to the movement of said carriage (1); an abutment decision device (23); a paper thickness calculation means (24) for calculating the thickness of recording paper on the basis of the movement of said carriage (1) from a reference position to a point of time a signal is produced from said abutment decision device (23); and a control means (20) for adjusting a relative gap (G) between said carriage (1) and said platen (4) by driving said stepping motor (5) in accordance with a data given by said paper thickness calculation means (24), characterized in that a time difference detection device (21) is provided for detecting whether, starting from said reference position, a change is arisen between the pulse signal period from the movement detection means (12) and the driving period of the stepping motor (5) and outputting a time difference ΔTn for each period n; a time difference integrating device (22) is provided for calculating the integrated value Σ(ΔTn) of the time differences ΔTn; and said abutment decision device (23) is provided for outputting a signal when the integrated value Σ(ΔTn) of the time differences ΔTn given by the time difference integrating device (22) coincides with a preset value. The automatic platen gap adjusting apparatus of claim 1 wherein the carriage (1) is moved perpendicularly to the platen shaft. The automatic platen gap adjusting apparatus of claim 1 or 2, further comprising a moving means for moving said carriage to a substantially central position of a printing region so that said relative gap (G) between said carriage (1) and said platen (4) is adjusted in said central position. The automatic platen gap adjusting apparatus of any one of the preceding claims, wherein an integration of said time difference is carried out after the movement of said carriage is stabilized. The automatic platen gap adjusting apparatus of any one of the preceding claims, wherein said movement detection means (12) outputs one pulse signal in synchronization with one driving pulse. The automatic platen gap adjusting apparatus of claim 5, wherein said predetermined value of said integrated time difference value for deciding an abutment state is not greater than 1/2 of a driving pulse period of said stepping motor. The automatic platen gap adjusting apparatus of any one of the preceding claims, further comprising an identifying means for indentifying the kind of recording paper from variations of said integrated time difference value with the passage of time.

Inventors: UCHIYAMA YUKIHIRO, UCHIYAMA, YUKIHIRO
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1994
Main classification: B41J25
IPC codes: B41J25